This invention relates generally to plasma etch reactors and, more particularly to techniques for controlling micro-contamination and the scavenging effect in such reactors. In a dry etching process typically used in the fabrication of integrated semiconductor devices, process gases are supplied to a reactor chamber and radio-frequency (rf) energy generates and sustains a plasma cloud within the chamber. Ions in the plasma cloud bombard a workpiece, which is usually a semiconductor wafer located in the chamber immediately adjacent to the plasma, or in a separate processing chamber into which ions from the plasma are drawn. The ions either etch the workpiece or assist the etching, and the etching process may be made selective by patterning a protective coating applied to the workpiece prior to etching. Selective etching requires the etch process to proceed at different rates for different materials, such as when a substrate material is etched in preference to a masking material. Some plasma particles, such as fluorine atoms, are notoriously non-preferential and spontaneously react with silicon and silicon dioxide, which is a common masking material. One solution to this difficulty is to use silicon as a scavenging electrode material, to absorb free fluorine in the plasma and thereby improve selectivity. However, an inherent difficulty with this process is that the scavenging effect is progressively reduced as the surface of the silicon electrode becomes coated with polymer materials or with by-products of the reaction with fluorine.
In general, there are three types of plasma generation approaches: capacitive, inductive, and microwave. In the more conventional capacitive plasma approach, the plasma is formed between a pair of parallel plate electrodes, to which radio-frequency (rf) power is applied, to one or both plates. A variant of the parallel plate approach is the magnetically enhanced reactive ion etch (MERIE) plasma generation apparatus, in which a magnetic field enhances the formation of ions in the plasma. Inductive plasma generators use an inductive coil, either a planar coil, a cylindrical coil or any of various other types of coils to deliver rf power into the plasma chamber. A separate rf generator supplies energy to at least one plate electrode in the chamber, to control ion energy and direction.
In a reactor chamber having a grounded upper electrode, referred to as the anode, the surface of the anode, including the exposed surface of the chamber, which is also grounded, is usually much larger than the surface of the lower electrode, the cathode. This results in a large direct current (dc) bias on the cathode and improves the plasma energy and the etch rate. However, the anode surface collects a polymer deposit during etching and the deposit can be a significant source and/or root cause of particle contamination. The deposit could also significantly affect the process conditions since the accumulated polymer alters the rf return impedance. In some reactors, this deposit could change the reaction chemistry, particularly the etch reaction, which depends upon the material of the top electrode. The mechanism resulting in particle contamination is not completely understood but may be explained as follows. As the polymer layer gets thicker, return rf current, which is normally distributed uniformly over the entire anode surface, flows instead through regions that are not coated with polymer. The only areas that have no polymer depositions are gas inlet holes in the anode, which serve to introduce process gases into the chamber. Increased current density around the gas holes provides a "current crowding effect" in those areas and electrical arcing may take place. The arcing produces molten aluminum particles. Particle contamination, either in the form of these aluminum particles or in the form of polymer flakes, is a likely result if the polymer layer becomes thick enough between cleanings. In configurations in which the top electrode has no gas inlet holes, the polymer deposition could significantly change the electrode surface chemistry reaction rate. As a result, the etch performance, such as selectivity or etch rates, may drift and cause production problems.
The conventional solution to this problem is to perform an in-situ dry cleaning process periodically, such as after processing each wafer or twenty-five wafers. For an oxide etching process, such dry cleaning is effective in controlling polymer deposition, eliminating gas hole arcing, controlling particle contamination and stabilizing etch performance, but is not without cost. Dry cleaning reduces processing through-put, increases the difficulty of automating processing, and shortens the useful life of "process kits," i.e., replaceable components installed in the chamber.
A similar problem arises in relation to an existing power-splitting version of a reactive ion etch chamber. In this device, the upper and lower electrodes are placed relatively close together and rf power from a single source is split equally between the electrodes, and applied with a phase difference of 180.degree.. This arrangement confines the resulting plasma between the electrodes but also results in erosion of the upper electrode, and consequently a shortened useful life of the electrode.
It will be appreciated from the foregoing that there is still a need for improvement in plasma etch techniques. Ideally, what is needed is a plasma etch system that avoids the problems outlined above and, in particular, a system that minimizes or avoids off-line dry cleaning to remove deposits from the top electrode, but does not result in substantial erosion of the electrode. The present invention satisfies this need.